The present invention relates generally to horizontally acting seat suspensions for vehicles and more particularly to a shock absorbing horizontal seat suspension for off-road vehicles.
In the past, prior art horizontal seat suspensions such as those typified by the U.S. Pat. Nos. to Smith (2,850,073), Simmons et al (2,932,324), Radke et al (3,100,617), Grizzle, Jr. (3,190,592), and Oswald (3,258,241) were simple spring mass systems which were intended only for on-road vehicles and use where horizontal vibrations are invariably above the natural frequency of the seat suspensions.
When the vehicle is driven off-road where the vibrations and shocks are routinely below the natural frequency of the seat suspension, conventional practice dictates the use of a lockout device to deactivate the simple spring mass system and to rigidly secure the seat to the vehicle. Of course this means that no vibration or shock attenuation is provided for off-road driving.
Attenuation of off-road vibrations and shocks has been a long standing, industry-wide problem and the problem has been especially acute in the agricultural industry where the vehicles are tractors and combines which are intended primarily for offroad use.
Only after extensive experimentation and analyses has it been determined that the prior art suspensions are unsuited and unmodifiable for off-road vehicles. A significant number of interrelated acceptable seat suspensions could be evolved.
In order to obtain maximum attenuation of vibration imposed on the operator in a spring mass system, a low spring rate spring is essential. However, a low spring rate spring alone presents the problems that small static loads or imputs at or below the natural frequency generate large displacements of the seat relative to the vehicle. Further, shock loads cause the spring mass system to resonate due to the inherent low centering forces acting to return the seat to the ideal centered position. These problems are unacceptable when the seated operator to vehicle controls distance must be maintained within plus or minus one inch (25.4 mm) from the ideal centered position.
Further modification and testing of prior art suspensions revealed that low spring rates result in the suspension working to one end of the displacement or continuously hitting displacement stop devices as the vehicle was driven on slopes or over rough, off-road terrain. A thorough analysis indicated that the problems arose in part from static loads, system friction, and directionally random low frequency vibrations and shocks which are all related to the low spring force due to the low spring rate. Attempts to add a shock absorber to alleviate some of the problems were unsuccessful, again because the low spring force would not center the seat to provide adequate travel for proper operation of the shock absorber. Attempts to reduce system friction with low friction bearings resulted in the spring mass system being subject to unacceptable resonance under shock loads and greater problems with the small static loads.
Since the prior art suspensions could not be modified to produce an acceptable horizontal seat suspension, an entirely new approach was used. The result is a totally new, complex spring mass system utilizing a pair of opposed, unidirectionally acting, preloaded springs. The new system provides positive centering forces while allowing low spring rate springs and prevents system resonance after shock loads.
To insure that the positive centering forces would center the seat, it was discovered that ultra low friction bearings were required in the seat displacement support to prevent binding under small static loads, system friction, and directionally random low frequency inputs.
Only after combining the features of the new spring system and the ultra low friction bearings was it possible to add a shock absorber to obtain acceptable and highly satisfactory performance of the suspension above and below the natural frequency of the spring mass system.